What stuff got doubled?
It looks like, in if (!empty($modSettings['onlineEnable']) && !$message['member']['is_guest'])
echo '
<li class="online">', $context['can_send_pm'] ? '<a href="' . $message['member']['online']['href'] . '" title="' . $message['member']['online']['label'] . '">' : '', '<img src="', $message['member']['online']['image_href'], '" alt="', $message['member']['online']['text'], '" />', $context['can_send_pm'] ? '</a>' : '', ' ', $message['member']['online']['text'], '</li>';
It is the 'href' which is not defined -- like something was left over in the template after you uninstalled a mod?